Plastic printing plate



5 1967 J. DE MARIA ETAL 3,338,163

PLASTIC PR1 NTING PLATE Filed Jan. 24, 1966 FIG. 3 -25 zsmw' '////7//// LAYER 23 THERMOPLASTTC GLASS FTBER MAT, EMBEDDED AT SURFACE \THERMOPLASTIC \EZwEQ RELEASE PAPER INVENTORS JOHN de MARIA EDWARD F. HOGAN ERNEST R. PEARSON KENWAY, JENNEY. WITTFR K. HLD

ATTORNEYS United States Patent 3,338,163 PLASTIC PRINTING PLATE John De Maria, Pond St, Rehoboth, Mass. 02769;

Ernest R. Pearson, 1 Pinecone Drive, West Barring ton, R1. 02890; and Edward F. Hogan, 31 Via Contenta, Carmel Valley, Calif. 93924 Filed Jan. 24, 1966, Ser. No. 534,265 5 Claims. (Cl. 101-4011) This application is a continuation-impart of application Ser. No. 46,815, filed Aug. 1, 1960, now abandoned. This invention relates to plastic printing plates and to an improved method of manufacture which provides plates of high quality on a commercially practical basis.

: In the past, electrotype printing plates have been used for high quality book printing and other uses where exact reproduction is required. These plates are made by coating a matrix with a thin film of silver, electroplating a layer of copper over the silver film, and then casting a metal backing to give the plate suflicient body to bev handled. The finished plate is then ground on the back to fit the press bed, and blank areas on the face may be routed out to eliminate the danger of contacting the paper in areas where no printing is to be provided.

Synthetic plastics and rubber are also commonly employed for printing plates, but their uses have so far been for the most part limited to relatively rough types of work such as printing on boxboard and in the manufacture of telephone books. Plastic and rubber plates are not generally used for high quality work such as required for book printing. The present invention provides an improved method of manufacture from which plates of controlled uniform thickness can be produced from readily molded or cast plastic compositions. In one aspect it provides a method by which a fluid hardenab le plastic composition, suitably hardfor forming a printing plate facing which bears the characters to be printed, may be combined into a plate and readily processed to a finished product of controlled uniform thickness. In another aspect this invention provides a pro-formed backing element which serves to facilitate the manufacture of plastic printing plates. In particular the pre-formed backing provides for a finishing operation by which plates of precision quality may be formed.

.In general, the invention makes use of a pre-formed backing sheet which serves to make up the bulk of the plate and to carry the characters of a previously cast plastic facing; By utilizing a casting operation to form the characters from a fluid hardenable plastic composition, a highly accurate reproduction of the matrix is formed. The facing is then backed up and built into the plate structure by pressing it to a. heat-softened pre-formed backing having a thermoplastic composition at its surface which contacts the top of the facing as it is supported in the matrix. Assembling the plate in this manner provides a high degree of precision in the thickness of the plate while retaining the quality of the cast facing.

The backing sheet of this invention is most preferably a composite structure having at one surface a layer of thermoplastic material which when softened is applied to the upper surface of the facing, and a layer at the opposite surface of a formulation which may be ground in the manner that a metal plate is ground to control the thickness further.

The layer of thermoplastic material is desirably resilient and ofsubstantially uniformth-ickness. The grindable layer is preferably substantially less resilient and of suflicient thickness that only parts of it, and no part of the resilient layer, are removed during the grinding operation.

In this manner the plate remains substantially uniformly resilient over its entire area, notwithstanding that mate- 3,338,163 Patented Aug. 29, 1967 rial has been ground away from its back side in a nonuniform'pattern. As the grindable material that was removed, as well as that remaining is non-resilient, the total resiliency (imparted by the thermoplastic layer) remains unchanged.

Where high strength and dimensional stability are required, the plates of this invention may incorporate a reinforcing structure which preferably comprises a web or mat of essentially unoriented fibers (e.g., a sheet of unwoven fibers of random orientation in the plane of the sheet). Such reinforcement permits the plate to be assembled and formed under molding pressure without creating internal stresses which would otherwise cause the plate to warp or buckle when the pressure is released.

The web or mat of unoriented glass fibers may be either assembled between the cast facing and the preformed backing as a separate element, or it may be incorporated in the backing as part of a composite structure. An advantage to assembling a separate web or mat between the facing and backing lies in the ease with which air is expelled from between these sheets through the void regions of the web or mat. The assembly operations are, however, greatly simplified if the web or mat is an integral part of the backing, in which case the face of the backing contacting the back of the facing may be roughened, knurled, or corrugated to facilitate the elimination of air from between the backing and facing.

The unwoven web or mat provides a high degree of reinforcement without introducing unwanted stresses that can cause distortion, as would result from woven fabrics, screens, and the like. Apparently the unoriented random arangement of fibers accommodates the plastic to flow by permitting the individual fibers to shift with the flow without a tendency to revert to their unstressed position. Where dimensional stability is not important, a reinforcement of woven fabric sheet material, or other strong sheet material, may be employed. With such, warping or distortion following relief of the pressure applied during assembly may be minimized if the thermoplastic backing is heated to bring it to a flowable plastic or viscous condition before the application of assembly pressure.

The presently preferred embodiment of this invention is described in detail below with reference to the accompanying drawings in which:

FIG. 1 is a fragmentary view in vertical section showing schematically the casting of a fluid plastic composition to a printing plate matrix;

FIG. 2 shows the application of a pre-formed backing sheet to the cast facing after it has been hardened;

FIG. 3 is a view partly in vertical section illustrating the construction of the backing layer;

FIG. 4is a view in vertical section showing the assembled backing layer, facing, and matrix being pressed between platens to cause the backing and facing to combine into an assembly of uniform thickness;

, FIG. 5 is a schematic view in transverse section showing the back of the printing plate, after removal from the matrix, being ground; and

FIG. 6 is a fragmentary view in cross-section of a baking in which a mat of unwoven glass fibers is embedded at the surface to be applied to the back of the facing.

The manufacture of printing plates according to this invention features first the casting of printing plate facing on a matrix. For this purpose fluid plastic compositions capable of being hardened without substantial dimensional change are employed. The preferred composition is of the type commonly known as a plastisol, that is to say a thermoplastic resin suspended in a plasticizer to form a paste which upon heating coalesces into a homogeneous plastic 'body. These compositions are generally formed of vinyl chloride polymers and copolymers and a plasticizer such as dioctyl phthalate. These are commercially available and those preferred for this invention are formulated to harden into a solid having a hardness of at least 100 on the Shore duromet-er A scale, and preferably at least 60 on the Shore durometer D scale.

In forming the facing, the matrix 10 is first covered with a quantity of the plastisol composition 12, which is then wiped over the surface of the matrix 10 by doctor blade 14 to cause complete wetting of the matrix surface and form a facing layer sufficiently thick to cover the top surface 16 of the matrix; After the facing has been cast, it is caused to solidify as by heating to about 350 F. for about minutes. A solid film 18 with characters 20 projecting from its lower surface is thus formed.

The backing layer 22 is laid over the facing film 18, preferably with metal tabs 24 projecting at the four corners frombetween the facing and backing to provide novel means for attaching the finished plate to the press.

The backing is a pre-formed sheet having a thermoplastic material at the surface in contact with the facing 18 and an opposite surface formulated to permit grinding by means of an abrasive wheel. The backing sheet 22 may be simply a sheet of flexible thermoplastic material which is laid over the cast facing 18 preferably with a web or mat 21 of unwoven unoriented glass fibers of the type commonly known as a facing mat. Such a backing sheet maybe formed simply by casting a vinyl plastisol on'a supporting base of release paper, and then heating the plastisol to cause it to gel to a thermoplastic solid sheet. The release paper is readily stripped free and the film may then be laid over the web or mat 21.

Where dimensional stability is not of paramount importance, the backing sheet 22 may conveniently be formed, as suggested by FIG. 3, by coating a plastisol to a fabric backing to form a thermoplastic layer 23 of about 0.020 to 0.050 inch in thickness, then heating the layer to cause it to harden. The opposite side of the fabric is then coated with a plastisol composition 25 formulated to permit grinding, to about the same thickness and it is heated to cause it to harden. Typically, the first coating will comprise a vinyl resin plastisol composition having a hardness when cured of at least about 60 on the Shore durometer A scale, and the composition formulated for grinding will be a similar plastisol containing from about 30 percent .to about 60 percent by volume of a frangible filler material such as fine beads, preferably hollow, of

phenolic resin, glass, or other similarly brittle material.

After the backing is laid over the facing film 20, the assembly is heated to cause the thermoplastic backing to soften sufiiciently that it may be combined under pressure to the facing. The above-described backing formed of a plastisol composition is suitably heated in an oven at 350 F. for about 5 minutes. The assembly is then pressed at about 2 tons/sq. in. between platens to the desired final thickness which is conveniently controlled by surrounding the assembly with bearing blocks 26 against which the platens 28 hear. The plate assembly is held in the mold until it has cooled substantially to room temperature. Upon removal the plate is stripped from the matrix and may then be mounted on the rotary bed 30 of a conventional plate grinder and further processing in conventional manner.

A plate formed in this way with the tabs 24 embedded at each of the four corners is readily mounted to a printing press bed, either rotary or flat, and may be held by the tabs 24 under tension.

A composite backing which combines the advantages of a pre-formed reinforced sheet of the type shown in FIG. 3 with the dimensional stability achieved by using a reinforcing web of unwoven unoriented fibers is shown in FIG. 6. A grindable thermoplastic layer 25a such as that described above with reference to FIG. 3 is first cast on a release paper support 29, and is heated to cause it to harden. A layer of a liquid vinyl plastisol composition 231: hardenable into a thermoplastic solid is then cast over the grindable layer 25a, a glass fiber web 30 is then immersed in the liquid adjacent to its upper surface, and the liquid layer 23a is heated and cured. The result is a pre-formed backing from which the release paper support 29 may be stripped. The composite sheet may then be laid over the cured cast facing '18, as shown in FIG. 2, but without the need for the reinforcing web 21, the function of which is substantially assumed by the web or mat 30 already present in the backing.

Forming the backing member from a plastisol offers numerous advantages. Frangible beads of brittle materials can readily be added because the composition is inie tially a liquid into which they may easily be mixed. Other.

plastics or rubber would require severe milling .operations which would tend to breakthe beads. If a reinforcing fabric is incorporated into the backing sheet, the plastisol flows between the fibers with little tendency to cause them to shift, as would occur with other plastic materials.

The final plate is of uniform characteristics and both durable and highly faithful. It will generally be formed of flexible plastic materials, and may thus be used for either rotary or flat bed work, and the presence of a cast facing and a pre-formed hot pressed uniformly thick backing gives it the necessary characteristics for fine printing work.

Although this invention has been described with refsrence to preferred embodiments employing plastisol compositions, it is contemplated that modifications of the composite plate structure will readily occur to those skilled in the art and familiar with this disclosure and that such can be made without departing from the scope of this invention.

Having thus disclosed our invention and described in detail preferred embodiments thereof, we claim and desire to secure by Letters Patent:

1. In a flexible plastic printing plate having a facing layer forming raised print characters; a flat surfaced flexible backing sheet bondedto and supporting said facing layer, comprising a first layer adjacent to said facing layer formed of a thermoplastic cured vinyl plastisol resin which is substantially free of filler material, and on the other side of said first layer a backing layer of thermoplastic cured vinyl plastisol resin containing from about 30 to about 60 percent by volume of a-filler of fine hollow frangible beads rendering said backing layer grindable.

2. A backing sheet as defined by claim 1 containing within said first layer a fabric reinforcement.

3. A backing sheet as defined by claim 2 wherein the fabric is a web of unwoven unoriented fibers.

4. A backing sheet as defined by claim 1 wherein the filler is glass beads.

5. A backing sheet as defined by claim 1 wherein the fill-er is beads of phenolic resin.

References Cited UNITED STATES PATENTS 2,700,630 1/1955 Bukey et a1. 2,806,812 9/1957 Merz 161-160 X 3,046,172 7/ 1962 Reid. 3,145,654 8/1964 Johnson et a1. 101--395 EARL M. BERGERT, Primary Examiner.

MORRIS SUSSMAN,'Examiner. I ALEXANDER WYMAN, Assistant Examiner. 

1. IN A FLEXIBLE PLASTIC PRINTING PLATE HAVING A FACING LAYER FORMING RAISED PRINT CHARACTERS; A FLAT SURFACED FLEXIBLE BACKING SHEET BONDED TO AND SUPPORTING SAID FACING LAYER, COMPRISING A FIRST LAYER ADJACENT TO SAID FACING LAYER FORMED OF A THERMOPLASTIC CURED VINYL PLASTISOL RESIN WHICH IS SUBSTANTIALLY FREE OF FILLER MATERIAL, AND ON THE OTHER SIDE OF SAID FIRST LAYER A BACKING LAYER OF THERMOPLASTIC CURED VINYL PLASTISOL RESIN CONTAINING FROM ABOUT 30 TO ABOUT 60 PERCENT BY VOLUME OF A FILLER OF FINE HOLLOW FRANGIBLE BEADS RENDERING SAID BACKING LAYER GRINDABLE. 